Many processes are available for manufacturing a tubular workpiece having a circular, oval or otherwise hollow cross section with a transition portion. Applications for these components include catalytic converter housings used in automotive exhaust systems. In the prior art, these components were usually made from several pieces, such as a pair of clam shells or a tubular section and formed end pieces joined by non-sophisticated techniques, such as resistance, TIG or MIG welding. However, welding these components together is not desirable because of durability concerns.
Other known processes for forming a transition portion on a work piece include forming techniques. One such technique is a ram forming process. However, ram forming has limitations regarding diameter reduction ratios. Another known process is spin forming, one example of an apparatus for spin forming is shown in FIGS. 1–4. A spin forming apparatus 1 of the prior art includes a plurality of rollers 3 supported by a rotatable carrier 2. Each roller 3 has a tapered face 4. The rollers 3 reduce the original diameter 12 of workpiece 6 to a reduced diameter 8. A mandrel 5 provides internal support to the workpiece 6 during a spin forming operation. Although the prior art spin forming apparatus disclosed in FIGS. 1–4 is effective for creating a transition portion on a workpiece, there are a number of shortcomings associated with the apparatus 1.
One shortcoming of apparatus 1 is the reduction ratio, the ratio of the original diameter to the reduced diameter, that can be achieved. Exceeding the reduction ratio limitation will collapse the reduced portion of the workpiece, resulting in scrap. The amount of reduction available for apparatus 1 is limited by the reduction ratio.
Another limitation inherent in apparatus 1 is multiple machines are required to achieve a desired reduction in diameter if multiple passes are required for additional reduction in diameter beyond the limitations of the reduction ratio for apparatus 1. Accordingly, the workpiece must be transferred from one machine to another machine that has rollers that are arranged in a smaller diameter to further reduce the diameter of a portion of a workpiece. The workpiece continues to be transferred to another machine having a smaller diameter yet, until the desired diameter is achieved. As a result, additional machines, or stations, are required as well as additional floor space. Furthermore, a significant amount of time is required to reduce a portion of the workpiece.
Other spin forming machines have rollers that are inwardly adjustable to permit multiple passes on a work piece by a single machine. This solution may eliminate the need for multiple machines to reduce the diameter of a single workpiece; however, these machines still have limitations in the reduction ratio for a single forming pass. Therefore, several passes are required to achieve a desired reduction in diameter of a workpiece. For example, 21 passes are typically required to reduce a portion of a workpiece from a 4 inch diameter to a 2 inch diameter. Although spin forming machines that have inwardly adjustable rollers respond to the concerns of floor space usage and multiple stations, these spin forming machines are still not efficient enough.
Referring now to FIG. 5, an improved spin forming apparatus 9 according to the prior art is shown. The apparatus 9 includes a plurality of rollers 11 operatively supported by a rotatably supported carrier 10. Each of the rollers 11 is radially and axially offset from the other rollers 11. The axial and radial offset of the rollers 11 allows the apparatus 9 to make multiple reductions in a single forming pass, resulting in a superior reduction ratio for a work piece. As workpiece 6 and rollers 11 are engaged, the one of the rollers 11 furthest from the carrier 10 will contact the workpiece 6 first. As the rollers 11 and workpiece 6 are further engaged, the next one of the rollers 11 closest to the carrier 10 will contact the workpiece 6, further reducing the workpiece 6. This process continues until the workpiece 6 and rollers 11 are completely engaged. Apparatus 9 provides a favorable reduction ratio and an improved forming time, however, multiple stations are still required, as apparatus 9 is limited by the number of rollers that may be mounted on the carrier 10. As an example, four stations would be required to reduce a workpiece from a 4 inch diameter to a 2 inch diameter by employing apparatus 9.
Therefore, there exists a need for a spin forming process and machine that does not require multiple stations. There further exists a need for a spin forming machine and process that has an improved efficiency.
Thus, it is desirable to provide a method and apparatus for spin forming a workpiece that has an improved efficiency while capable of completing a forming operation on a single machine.